1. Field of the Invention
The present invention relates to rotary drum filters and particularly to those of the pipe machine type having an axial annular valve housing at one end of the drum. More specifically, the invention relates to a method of making a substantially entirely plastic rotary drum filter and to a method and mold for making an annular valve housing for such a drum filter.
2. Description of the Prior Art
Rotary drum filters of the pipe machine type are commonly used in the pulp and paper making industry to separate wood pulp from its filtrate. A drum filter comprises a rotary drum partially submerged in a tank of pulp slurry. The drum has axially extending filtrate channels or compartments spaced abouts its periphery, with such compartments covered by a filter screen. These filtrate compartments communicate through bucket sections, drainage conduits, and an annular valve housing in communication with a source of subatmospheric pressure. A stationary valve positioned within a valve seat portion of the valve housing controls the application of subatmospheric pressure to the respective valve chambers of the housing. As the drum rotates abouts its axis with subatmospheric pressure applied, the screen rotates through the pulp slurry and collects a wet mat of fibers from the slurry. As the screen emerges from the tank, filtrate is drawn through the screen and compartments into the bucket sections. From the bucket sections the filtrate passes through drainage conduits and valve chambers from where it is discharged through suitable piping from the drum and thus liquid is removed from the mat. As the drum continues its rotation, the stationary valve member periodically blocks off the filtrate compartments from the source of subatmospheric pressure thereby enabling removal of the pulp mat from the surface screen.
A conventional pipe machine type rotary drum filter as described above is shown in U.S. Pat. No. 3,363,774 of Luthi. Heretofore, such rotary filter drums and their components have commonly been constructed virtually entirely of metal such as stainless steel or titanium. These metal drums are relatively much more expensive and are more time consuming to construct in comparison with drum filters of the present invention. Furthermore, these metal components tend to rapidly wear because of their continual exposure to highly abrasive and often chemically active filtrate liquids, thereby greatly reducing the operating life of such drums.
In attempts to partially solve this corrosion problem, rotary drum filters have been manufactured with certain components of plastic, and particularly those components attached to the outer surface of the deck of the drum. U.S. Pat. Nos. 3,577,315 of Franklin, 3,579,422 of Minick, et al. and 3,794,178 of Luthi, as well as the above-mentioned Luthi patent exemplify this approach. Nevertheless, insofar as known, such drums include substantial metallic sections which are subject to corrosion.
Also, because the metallic sections are usually integrated with the structural members of the drum, when they wear out, the entire drum must be taken out of service and either replaced with a new drum or shipped to a maintenance facility for repair. Whether the drum is replaced or repaired, either choice is expensive and entails considerable lost production time.
In addition, during the course of repairing such drums, it has been known to clean, refurbish, and coat the corroded metallic surfaces with a plastic material to shield them from the corrosive filtrate liquids after the repair process is complete. However, composite drums of metal coated with a corrosion resistant plastic are somewhat costly to produce. Furthermore, cracks in the plastic coating can result during operation of the drum. This allows the corrosive filtrate to reach the metal components and constribute to the breakdown and subsequent repair of the drum. Drums of the type disclosed in the above-mentioned Luthi patents have been repaired in this manner.
In addition, U.S. Pat. No. 3,894,899 of Konopatov, et al. mentions a honeycombless rotary drum filter in which binder impregnated glass fabric is used in its manufacture. However, such a drum differs structurally from the drum of the present invention and is understood to include a central metal axle extending between its ends. In addition, the method of manufacturing such a drum is understood to differ from the method of the present invention.
Furthermore, none of the aforementioned devices are understood to include a plastic annular valve housing with plural valve chambers. In contrast, the annular valve housings in Luthi are understood to be of cast metal. As result, the surfaces are apparently more porous than the surface of the plastic annular valve housing resulting from the method of this invention. This greater porousity results in more surfaces on which the corrosive filtrate liquid can work and makes them wear faster.
Accordingly, there is a need for an efficient method of manufacturing a plastic rotary drum filter, particularly one of the pipe machine type. Furthermore, there is a need for a method of manufacturing a plastic annular valve housing which is segmented into plural valve chambers.